Expansion machines operative to generate power by expansion of high-pressure fluids, such as a displacement type expansion machine (e.g., a rotary type expansion machine), have been known in the conventional technology (see for example Japanese Patent Application Kokai Publication No. 1996-338356). Such a type of expansion machine can be used to perform for example a vapor compression refrigerating cycle's expansion stroke.
The expansion machine described above includes a cylinder and a piston which revolves along the inner peripheral surface of the cylinder. An expansion chamber, defined between the cylinder and the piston, is divided into two compartments, namely a suction/expansion side and a discharge side. And, as the piston revolves, a portion serving as the suction/expansion side switches to the discharge side while another portion serving as the discharge side switches to the suction/expansion side, such switching taking place in sequence in the expansion chamber. Accordingly, the action of high-pressure fluid's suction/expansion is carried out simultaneously concurrently with the action of high-pressure fluid's discharge.
In the above-descried expansion machine, both the angular range of a suction process in which a supply of high-pressure fluid is provided to the inside of the cylinder during a single revolution of the piston and the angular range of an expansion process in which fluid expansion is carried out are predetermined. In other words, generally such a type of expansion machine has a constant expansion ratio (i.e., density ratio of sucked refrigerant and discharged refrigerant). And, a high-pressure fluid is introduced into the cylinder in the angular range of the suction process while on the other hand the fluid is expanded at the fixed expansion ratio in the remaining angular range of the expansion process for the recovery of rotational power.